1. Field of the Invention
The present invention relates generally to catalytic systems in which metal catalysts are attached to an oxide ceramic support which is in turn supported on a monolith structure. More particularly, the present invention involves improving such catalytic systems by increasing the adhesion between the metal particles and the ceramic support and/or increasing the adhesion between the ceramic support and the monolith structure. This increase in adhesion extends the useful catalytic lifetime of the system.
2. Description of Related Art
Metallic catalysts are used in large numbers of systems to catalyze a wide variety of chemical reactions. For example, nickel is used to catalyze methanation reactions where natural gas is created from carbon monoxide and hydrogen. Palladium is widely used in catalytic converters for automobile exhaust systems. Palladium is also used as a hydrogenation catalyst in numerous processes for creating a wide variety of specialty chemicals including alcohols, polymers, and food products. Platinum is also a popular metal catalyst that is used extensively in oil refining and fuel cells. Platinum has also been used widely in catalytic converters for exhaust systems.
Catalysts that promote the oxidation of unburned hydrocarbons and carbon monoxide, as well as reducing nitrogen oxides, have been widely used in treating the exhaust gas streams of internal combustion engines. Such catalysts are referred to as “three-way” conversion catalysts (TWC). TWC catalysts typically include various combinations of platinum, palladium and rhodium that are located on high surface area ceramic supports that are composed of one or more refractory oxides.
It is important in any catalytic system that the surface area of metallic catalyst that is exposed to the reactants be maximized. One way to obtain relatively large surface area exposure is to use metallic catalysts that are in the form of small particles. The catalytic particles are usually immobilized on the ceramic or refractory oxide support material. Ceramic supports that utilize alumina as the principal refractory oxide have been very popular. The ceramic supports are typically provided as powders that are coated onto a macro sized carrier particle or other type of monolithic supporting structure. Monolithic support structures are made from a wide variety of materials including ceramic and metal materials.
An important consideration in heterogeneous catalytic systems that combine ceramic supports with metal particles is to insure that the exposed surface area of the metal catalyst does not diminish over time. This is especially important for catalytic systems that operate at high temperatures and utilize small metallic particles attached to a ceramic support. Sintering tends to occur in these types of systems that results in agglomeration of the particles and substantial reductions in exposed metallic surface areas. This reduction in exposed surface area substantially reduces the activity of the catalyst particles and results in decreased lifetime of the catalytic system. In addition, it is important that the ceramic support remain firmly attached to the monolith structure. This is a particular problem when the ceramic support is attached to a metallic monolith.
Numerous attempts have been made over the years to maintain high exposure of metallic catalyst surface area. Most of these attempts have focused on modifying the properties of the ceramic support to “stabilize” the support and prevent occlusion of the metallic catalyst. For example, U.S. Pat. No. 4,171,288 describes stabilizing alumina supports by adding zirconia, titania, alkaline earth metal oxides, such as baria, calcia or strontia or rare earth metal oxides, such are ceria, lanthana and mixtures of two or more rare earth metal oxides. Other examples of alumina supports that have been modified to increase activity and durability are described in U.S. Pat. Nos. 3,966,790; 4,294,726; 4,624,940; 4,714,694; 4,849,399; 4,902,664; 5,212,142; 5,552,360; 5,627,124; and 6,492,297. The contents of the preceding patents are hereby specifically incorporated by reference.
Although many of the above catalytic systems work well for their intended purposes, there is a continuing need to develop new systems and methods that can be used in a wide variety of heterogeneous metallic catalyst systems to maintain high catalytic metal surface areas and good adhesion between the ceramic support and underlying monolith.